1. Field of the Invention
The present invention relates to an image signal resolution changing system that changes the resolution of image signals read by an image reading device or the like.
2. Description of Related Art
An example of an image input unit in one type of a conventional image input system is shown in FIG. 5. Light passes through or is reflected from an original document so that the light contains image information. As shown in FIG. 5, the input light image is received by a CCD line sensor 11, which performs photoelectric conversion of the light image such that the CCD line sensor 11 outputs electrical (analog) signals. Typically, the CCD line sensor 11 includes a line of photoelectric conversion elements extending in a primary scanning direction, whereby the CCD line sensor 11 outputs data one line at a time. The CCD line sensor 11 and the original document move relative to each other in the secondary scanning direction so that successive lines of data are output, representing the entire original document. Following this, the signals are digitized by an A/D converter 12, which outputs, for example, an 8-bit signal for each photoelectric sensor element. The digital image signals output by A/D converter 12 are temporarily stored in a buffer 13 (FIFO memory). The image signals that are stored in the buffer 13 are sent to a memory 14 via a bus line 16, the sending of the signals to the memory 14 being controlled by commands from the CPU 15.
With this kind of system, an image signal with the maximum resolution is achieved when all data (image signals) output from all photoelectric conversion elements in the CCD line sensor 11 are used. However, when a resolution less than the maximum resolution is to be obtained, the image signals output from the CCD line sensor 11 are thinned out. An example of thinning out is shown in FIGS. 6A-6D. FIGS. 6A-6D show an example of changing the resolution by a factor of 4. As shown in FIGS. 6A-6D, a resolution one-fourth the maximum resolution is realized by using the data (image signals) output by every fourth one of the photoelectric conversion elements in the CCD line sensor 11. That is to say, thinning out is accomplished by using only the data (image signals) from pixels (i.e., from photoelectric conversion elements) having a pixel number of the form 1+4n (n comprising an integer) in FIGS. 6A-6D. In order to realize a resolution one-half the maximum resolution, in a case which is not shown in the figure, thinning out would be similarly accomplished using only the data (image signals) from pixels having a pixel number of the form 1+2n (n comprising an integer) in the figure.
FIG. 6A illustrates an image comprised of four distinct light lines a, b, c and d. FIG. 6B shows the individual photoelectric conversion elements in the CCD line sensor 11. FIG. 6C shows the output of each photoelectric conversion element at maximum resolution. FIG. 6D shows the data resulting from the FIG. 6C CCD output after a 1/4 thinning process. With the conventional simple thinning out process shown in FIGS. 6A-6D, the problem exists that some input image light lines received by the CCD pixels are not reflected at all in the output. In other words, even though four slit light lines a, b, c and d (see FIG. 6A) exist in the input image, the data (image signals) corresponding to slit light lines a, c, and d are lost. Consequently, an image signal that is unlike the input image light lines is obtained.
Thus, there is a need for a system and method for reducing the resolution without losing useful image data.
The previous example demonstrated problems associated with conventional resolution reducing operations in which the resolution was reduced in the primary scanning direction. It also is known to reduce the resolution in the secondary scanning direction using a thinning process.
As detailed above, when reading-in the original document, movement is accomplished in the secondary scanning direction by means of a stepping motor, which moves the CCD line sensor 11, the original document, or both in the secondary scanning direction. After reading-in one line of data in the primary scanning direction, movement is accomplished in the secondary scanning direction, and then another line of data is read. This process repeats until the entire image is read.
FIG. 10 illustrates an example in which the resolution in the secondary scanning direction is reduced to one-fourth that of the maximum resolution. In FIG. 10, each item 1A-4D represents a line of data output by the CCD line sensor. That is, due to primary scanning direction scanning, a first line of data 1A is produced. Then, after relative movement in the secondary scanning direction, a second line of data 1B is output. This successive scanning and movement continues until the entire image is read.
With the simple thinning process accomplished in accordance with the prior art, and as shown in FIG. 10, the data from only one in every four lines of data is used. Accordingly, the input image data from a line that has not been used causes a problem in that there is absolutely nothing which is reflected back to the output signal. In other words, when the output of the CCD line sensor is comprised of lines 1A, 2A, 3A, . . . and when the output of the CCD line sensor is read-in in a proportion of one time per four steps, then there is the problem that the picture image input within the lines 1B, 1C, 1D, 2B, 2C, 2D, . . . etc. is ignored.